This invention relates to systems for converting flux transitions on moving magnetic media to binary data, and more particularly to systems for converting flux transitions in magnetic tape systems. Even more particularly, this invention relates to systems for detecting the presence of signals of sufficient amplitude to allow conversion of the flux transitions to binary data in a digital recording magnetic tape system.
The recording channel of a magnetic tape device presents unique detection requirements which are not found in devices with sealed media or constant speed such as disk drives. The magnetic coating of the tape is susceptible to drop-outs and wide variations in amplitude due to the type of coating, wear, and the length of time since the signal was placed on the tape. In addition, the amplitude varies as a function of the velocity of the tape past the read head and the amplitude also varies with the recording density. In a typical magnetic tape system, three different drive speeds are used, and three different recording densities are also used, resulting in nine combinations of speed and density found in the same read channel.
When data is written on magnetic tape, a read-back check is made using a separate read head, located behind the write head. During this read-back check, the signal amplitude requirement is higher than required during normal data reading, to ensure that the data will still be readable as the magnetic field on the tape degrades over time. This requirement adds two more combinations of amplitude, resulting in eighteen different combinations of read/write, speed and density which must be considered in determining if the proper amplitude is present at the output of the magnetic read head.
Problems also occur if the magnetic read/write head fails and must be replaced. Because of material and manufacturing variations, each head will have a different characteristic signal output level. This level must be normalized by adjusting the head amplifier gain so that the amplitude detection circuits will have the same signal margin. In order to properly perform this normalization adjustment, a specially recorded tape, with a precisely known recorded amplitude, must be used.
In prior art systems, these problems are solved by providing a gain adjustment on the amplifier, which must be readjusted periodically, and by providing different signal amplitude detection threshold levels for each combination of read/write, speed and density. These threshold levels were fixed, however, and could not change to adjust for media coating type, wear, or signal degradation over time. Also, these fixed threshold values required that the output of each magnetic head be normalized very precisely by adjusting its amplifier gain before amplitude detection can be reliable.
Amplitude sensing is an important part of the read circuitry of tape systems for digital data recording which record multiple tracks in parallel across the tape. In this type of system, error detection and correction methods are used extensively, such as parity checking across the parallel tracks or parity checking of the data bits in a single track. Loss of amplitude is an important indicator that a track is in error, and the correction methods can then be used to correct the track to avoid having to reposition and read the data again.